Weathering process at the natural fission reactor of Bangombe

The uranium deposits in the basin of Franceville (Gabon) host the only natu ral reactors known in the world. Unique geological conditions favoured a na tural fission reaction 2 Ga ago. This was detected by anomalous isotopic co mpositions of uranium and rare earth elements (REE), which are produced by the fission reaction. In total, 16 reactor zones were found. Most of them a re mined out. The reactor zone of Bangombe, is only 10-11 m below the surfa ce. This site has been influenced by surface weathering processes. Six dril l have been sampled at the site of the reactor zone of Bangombe during the course of the study and only one drill core (BAX 08) hit the core of the re actor. From these data and previous drilling campaigns, the reactor size is estimated to be 10 cm thick, 2-3 m wide and 4-6 m long. The migration of f ission products can be traced by the anomalous isotope ratios of REE becaus e of the fission process. The Sm-149/Sm-147 ratio close to the reactor zone is only 0.28 (normal: 0.92) because of the intense neu-capture of Sm-149 a nd subsequent transmutation, whereas Sm-147 is enriched by the fission reac tion. Similar changes in isotopic patterns are detectable on other REE. The isotope ratios of Sm and Nd of whole rock and fracture samples surrounding the reactor indicate that fission-genic REE migrated only a few decimetres above and mainly below the reactor zone. Organic matter (bi-tumen) seems t o act as a trap for fission-genic REE. Additional REE-patterns show less in tense weathering with increasing depth in the log profile and support a sim ple weathering model.